Really stupid question: When checking resistors while restoring an older
receiver, is it common to attempt to check the resistor while it is in the
circuit? That is, make a guesstimate of the parallel resistance (working
from the schematic). Or, should I disconnect one end of each resistor to
check it?

(Am working on an S-85. Have replaced most capacitors. No hum and the B+
voltages are OK, so the original power supply capacitors seem OK. General
sensitivity is still low, so resistor checks seem to be next on the list. IF
and RF alignment are OK. The last owner had the local osc above the signal
on band 4, which was the only serious problem. Tubes are OK, I think.)

Why an S-85? It was my first receiver, way back when (with a Heathkit Q
multiplier).

Bill - W2WO

"Bill Ogden" wrote in message
...
Really stupid question: When checking resistors while
restoring an older receiver, is it common to attempt to
check the resistor while it is in the circuit? That is,
make a guesstimate of the parallel resistance (working
from the schematic). Or, should I disconnect one end of
each resistor to check it?

(Am working on an S-85. Have replaced most capacitors. No
hum and the B+ voltages are OK, so the original power
supply capacitors seem OK. General sensitivity is still
low, so resistor checks seem to be next on the list. IF
and RF alignment are OK. The last owner had the local osc
above the signal on band 4, which was the only serious
problem. Tubes are OK, I think.)

Why an S-85? It was my first receiver, way back when (with
a Heathkit Q multiplier).

Bill - W2WO
It depends on the circuit. Often even if there is a
parallel resistance path you can tell if a resistor is very
far from its value without lifting it but an accurate
measurement usually does require lifting one end. Carbon
composistion resistors are not very stable so its common to
find very old ones which are way out of spec.


--
---
Richard Knoppow
Los Angeles, CA, USA

Bill Ogden wrote:
Really stupid question: When checking resistors while restoring an older
receiver, is it common to attempt to check the resistor while it is in the
circuit? That is, make a guesstimate of the parallel resistance (working
from the schematic). Or, should I disconnect one end of each resistor to
check it?

If you check it in-circuit, it will read at or lower than the resistor
value. It could be _lower_ because there is other stuff in the circuit
in parallel with the resistor. It could be _the same_ because that stuff
may not have any DC resistance when the thing isn't powered up. It will
not ever be higher unless the resistor is bad.

Since most resistors fail into a higher than normal value, or open, rather
than into a short, measuring in-circuit gives you a good quick way to find
a large proportion of bad resistors. But it won't find all of them.

(Am working on an S-85. Have replaced most capacitors. No hum and the B+
voltages are OK, so the original power supply capacitors seem OK. General
sensitivity is still low, so resistor checks seem to be next on the list. IF
and RF alignment are OK. The last owner had the local osc above the signal
on band 4, which was the only serious problem. Tubes are OK, I think.)

Where did you check the B+ voltages? I would be apt to check plate and
cathode voltages at each tube and make sure they are roughly sane.

Why an S-85? It was my first receiver, way back when (with a Heathkit Q
multiplier).

Well, why not an S-85?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 8/2/07 7:58 AM, in article , "Bill Ogden"
wrote:

Really stupid question: When checking resistors while restoring an older
receiver, is it common to attempt to check the resistor while it is in the
circuit? That is, make a guesstimate of the parallel resistance (working
from the schematic). Or, should I disconnect one end of each resistor to
check it?

(Am working on an S-85. Have replaced most capacitors. No hum and the B+
voltages are OK, so the original power supply capacitors seem OK. General
sensitivity is still low, so resistor checks seem to be next on the list. IF
and RF alignment are OK. The last owner had the local osc above the signal
on band 4, which was the only serious problem. Tubes are OK, I think.)

Why an S-85? It was my first receiver, way back when (with a Heathkit Q
multiplier).

Bill - W2WO



I'd be inclined to check the voltages on the tubes with the receiver
operating. Compare the readings with the ones in the manual. Be sure to
use a high resistance voltmeter so you don't load the test points and get
bad readings. No 20k Ohms per volt boatanchor meters.

Don

I agree with voltage checks at the actual tube socket with a good, very good,
i.e. high impedance [FET or equiv], voltmeter. If voltages are within 5% of
correct values, then the resistors should be within spec. If voltages are 15%
off, then replace.

With the S-85 you can remove the tubes which opens one side of the tube bias
resistors. So, with the tube removed plate decoupling resistors, screen grid
resistors, cathode resistors can be checked in circuit.

Secondarily, typical A-B carbon composition resistors absorb water vapor and
swell/crack with time/age. This is easily found by an eyeball exam with a little
magnification for those of us over 60 years old :-).

Finally, use a good contact cleaner on the bandswitch wafers. You can use a
Q-tip for those hard to reach places.

/s/ DD, W1MCE

Bill Ogden wrote:

Really stupid question: When checking resistors while restoring an older
receiver, is it common to attempt to check the resistor while it is in the
circuit? That is, make a guesstimate of the parallel resistance (working
from the schematic). Or, should I disconnect one end of each resistor to
check it?

(Am working on an S-85. Have replaced most capacitors. No hum and the B+
voltages are OK, so the original power supply capacitors seem OK. General
sensitivity is still low, so resistor checks seem to be next on the list. IF
and RF alignment are OK. The last owner had the local osc above the signal
on band 4, which was the only serious problem. Tubes are OK, I think.)

Why an S-85? It was my first receiver, way back when (with a Heathkit Q
multiplier).

Bill - W2WO

"David Shrader" wrote in message
. ..
I agree with voltage checks at the actual tube socket with
a good, very good, i.e. high impedance [FET or equiv],
voltmeter. If voltages are within 5% of correct values,
then the resistors should be within spec. If voltages are
15% off, then replace.

With the S-85 you can remove the tubes which opens one
side of the tube bias resistors. So, with the tube removed
plate decoupling resistors, screen grid resistors, cathode
resistors can be checked in circuit.

Secondarily, typical A-B carbon composition resistors
absorb water vapor and swell/crack with time/age. This is
easily found by an eyeball exam with a little
magnification for those of us over 60 years old :-).

Finally, use a good contact cleaner on the bandswitch
wafers. You can use a Q-tip for those hard to reach
places.

/s/ DD, W1MCE

snipping here...

Check the instruction book to see if it specifies the
type of meter used to get the readings in the chart. Very
often older receiver voltages were measured with low
resistance meters, around 1000 oms per volt. If you use a
modern very high resistance meter the voltages will read
high. For example, nearly all the specified voltages for the
SP-600-JX were measured with a 1000 ohm/volt meter except a
few which specify a VTVM.


--
---
Richard Knoppow
Los Angeles, CA, USA

Bill:

I'd suspect screen bypasses and cathode bypasses if the receiver is very
old. By all means disconnect one end of a resistor unless it connects to a
tube in which case you can pull the tube while measuring. If two resistors
are in parallel, the other one could be faulty , givign you a false
measurment.

Ralph VE3BBM
"Bill Ogden" wrote in message
...
Really stupid question: When checking resistors while restoring an older
receiver, is it common to attempt to check the resistor while it is in the
circuit? That is, make a guesstimate of the parallel resistance (working
from the schematic). Or, should I disconnect one end of each resistor to
check it?

(Am working on an S-85. Have replaced most capacitors. No hum and the B+
voltages are OK, so the original power supply capacitors seem OK. General
sensitivity is still low, so resistor checks seem to be next on the list.
IF and RF alignment are OK. The last owner had the local osc above the
signal on band 4, which was the only serious problem. Tubes are OK, I
think.)

Why an S-85? It was my first receiver, way back when (with a Heathkit Q
multiplier).

Bill - W2WO

On 8/2/07 8:26 AM, in article , "Scott Dorsey"
wrote:

(snip)


Where did you check the B+ voltages? I would be apt to check plate and
cathode voltages at each tube and make sure they are roughly sane.

And check the screen voltages and the control grid voltages.

(snip)

Thank you all for the responses. I will work with the following points:

1. Determine if the manual specifies the nature of the VOM/VTVM used to read
voltages. IIRC, many manuals of that era specified 20K ohms/volt meters ---
but some manuals did not specify anything at all.
2. If a resistor starts going bad, it usually increases in resistance. I
did not know this, and it is a very useful bit of information. As mentioned,
if an in-circuit resistor measurement is much higher than the nominal value,
then there is definitely something wrong.
3. Consider pulling tubes to effectively "lift" the ends of some resistors,
making measurements easy.
4. If nothing else works, lift one end of a few resistors.
5. I am over 60 also, and using a glass to inspect the resistors could be
interesting.

Regards,

Bill - W2WO

Thank you all for the responses. I will work with the following points:
5. I am over 60 also, and using a glass to inspect the resistors could be
interesting.

You might consider a headworn magnifier. There are some very inexpensive
ones sold by guys like Parts Express. I always pooh poohed them, but then
I actually tried one and it makes work MUCH easier. Also, a very strong
bench light helps a lot. You may not realize how much easier it is to work
with a very strong light until you try it.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."